Microplastic pollution is a growing environmental concern. Microplastics are plastic particles with sizes ranging from 1 μm to 5 mm. However, a clear consensus on the definition of microplastics is lacking and even the lower size limit varies among different studies. Microplastics originate from various sources such as industrial production, textile, packaging, and plastics products. The predominant sources of microplastics are the fragmentation of larger plastics.
A review by Zhang et al. (2020b) concluded that climate change will directly impact microplastic pollution due to the increased amounts of plastics entering the lakes through the increased precipitation and surface runoff. Furthermore, due to limited or lack of access to proper waste disposal conveniences in remote areas, the microplastics are discharged into freshwater and eventually settle in sediments.
Among freshwater sources, lakes are relatively closed water systems with long retention times, which can potentially become long-term sinks of microplastics. Microplastics research reported that microplastics are found in lake water, lake bottom sediment, and lakeshore sediment.
However, field-based microplastic studies in high-altitude lakes are lacking in comparison with microplastic studies in rivers. In such a scenario, freshwater in the Himalayan regions is an important site for microplastic research due to the lack of field-based microplastic data and the region's vulnerability to the repercussion of climate change.
Microplastic pollution in lakes in the Indian Himalayas has not been investigated except in the bottom sediment of Anchar Lake in Kashmir valley. A recent paper ‘Variation of microplastics in the shore sediment of high-altitude lakes of the Indian Himalayas using different pre-treatment methods’ deals with the issue of microplastic pollution in the Himalayan lakes.
According to the current state of microplastic research, direct comparison among microplastic abundance is difficult mainly due to different methods applied during sampling, pretreatment, and analysis. However, towards a holistic understanding of microplastic pollution and environmental monitoring, microplastic data with reliable comparability is important. Thus, the need for the harmonization of methods is widely recommended in microplastic research.
In this study, microplastic pollution was assessed in the shore sediment of three high-altitude lakes in Ladakh of the Indian Himalayas, namely Pangong Lake, Tsomoriri Lake, and Tsokar Lake. A sampling of lakes' shore sediment was performed in August 2019. Two different pretreatment methods were implemented with sediment samples from the same sites, resulting in two sets of samples.
One set of samples was pretreated utilizing enzymatic degradation together with Fenton reactions. Another set of samples from the same sites were pretreated with 30 % hydrogen peroxide (H2O2) and a Fenton reaction.
Enzymatically pretreated samples resulted in higher microplastic concentrations than the set of H2O2 pretreated samples, which indicated that microplastic concentrations in sediment samples vary even among samples from the same site and that the pretreatment procedure may impact the reported microplastic concentrations.
Considering both sets of samples, microplastic concentration was 160–1000 MP/kg dw in Pangong Lake, 960–3800 MP/kg dw in Tsomoriri Lake and 160–1000 MP/kg dw in Tsokar Lake. A blank correction based on the limit of detection and the limit of quantification indicated that microplastic concentrations at some sites of the studied lakes are higher than the limit of detection and the limit of quantification.
In this light, three brackish lakes on the Ladakh Himalaya were studied to investigate the distribution and abundance of microplastics in lake shore sediments. The hypothesis was that microplastics accumulate in the mountainous lakes of the Indian Himalayas, particularly in the shore sediment of lakes in Ladakh.
To contribute toward understanding the complexity of microplastics in the environment, microplastic abundances in the sediment samples from the same sites were investigated by using two different pretreatment methods including an enzymatic pretreatment method and hydrogen peroxide pretreatment method accompanied by Fenton reactions in both methods. Additionally, the study aims to address the harmonization of the microplastics research by applying a strict blank correction as suggested by Horton et al. (2021).
This study did not focus on a detailed assessment of the pathways and fate of microplastics. However, the potential pathways and fate of microplastics in the studied lakes were discussed.
A review by Padha et al. (2022) on the microplastic pollution in mountain terrains concluded that glacial retreat contributes to microplastics in rivers and lakes. Furthermore, microplastics were found in glaciers such as Mount Everest at an elevation of around 8850 m. Similarly, lakes in this study are located at an elevation above 4000 m and lake water is formed from the glaciers. Thus, glaciers retreat could contribute to microplastics in studied lakes.
A microplastics study by Allen et al. (2019) in French Pyrenees, a remote pristine mountain catchment, indicated that dry and wet deposition can be a potential source of microplastics in remote areas.
Precipitation in the Ladakh Himalaya is relatively low with an annual rainfall of around 100 mm, but seasonal fluctuation due to climate change impacts the frequency of precipitation over the years. Thus, precipitation could be also a pathway of microplastics into the studied lakes. In addition, aerial transport via wind could be another possibility of the microplastic pathway.
Surface runoff caused by the melting of snow, washing away plastic litter by rain, and cleaning of vehicles along the shore particularly in Pangong Lake and Tsomoriri Lake could add microplastics into the Lakeshore sediment.
Anthropogenic activities, including tourism such as trekking, vehicles, clothes with synthetic fibers, tents, and disposal of plastics litters like drinking water bottles and food packaging, around the lake sites could be among the major contributors to microplastics in studied lakes. Thus, proper waste management and environmentally sustainable tourism activities should be practiced.
Microplastics act as a vector for toxic contaminants such as heavy metals and organic pollutants that could intensify the negative impact of these contaminants on organisms, humans, and the surrounding environment.
Furthermore, lakes on the Ladakh Himalaya are enclosed with a high retention time of water with water exchange mainly through evaporation. The retention of microplastics in the studied lake sediment can be long and these lakes can act as a potential sink of microplastics.
Conclusion
According to this study, the shore sediments of lakes in the Indian Himalayas, including Pangong Lake, Tsomoriri Lake, and Tsokar Lake, have microplastic concentrations ranging from 160 to 3800 MP/kg dw. The maximum microplastic concentrations were observed in Tsomoriri Lake. The most dominant polymer compositions in all the studied sites were PE and PP. Fragmentation of larger plastic materials was potentially the main source of microplastics in the studied sites.
A significant difference in microplastic concentrations of sediment samples from the same site pretreated with two different methods demonstrated both the heterogeneity of microplastics in the sediment matrix and the effect of the pretreatment method on the resulting microplastic concentrations.
Furthermore, the findings highlighted the importance of reporting complete information in microplastic studies, especially during comparison among different studies of the same sample type. The study also demonstrated the importance of conducting procedural blanks and reporting microplastic concentrations with and without blank correction.
Overall, this research indicated that lakes in the Indian Himalayas are contaminated with microplastics. The lack of environmental sustainability strategy with growing tourism activities worsens the microplastic pollution, especially through improper waste treatment. Thus, strategies to combat plastic pollution in these regions must be implemented. This study further demonstrated that the high-altitude mountainous lakes can potentially become long-term sinks of microplastics.
In addition, the comparison of microplastics using different pretreatment methods illustrated the importance of harmonization of microplastic studies to enable a reliable comparison among microplastic data.
The full paper can be viewed here